US20140026498A1

US20140026498A1 - Antiseismic support

Info

Publication number: US20140026498A1
Application number: US13/985,425
Authority: US
Inventors: Virginio Quaglini; Daniele Cuminetti; Paolo Dubini; Riccardo Motta; Daniela Ferroni
Original assignee: Politecnico di Milano
Current assignee: Politecnico di Milano
Priority date: 2011-02-21
Filing date: 2012-02-20
Publication date: 2014-01-30
Also published as: EP2678496A1; IT1404858B1; ITMI20110257A1; WO2012114246A1

Abstract

The present invention concerns an anti-seismic support for supporting a construction engineering structure, which comprises a support (14) having a concave sliding surface (17) and a sliding block (16) with a convex sliding surface (16d) in sliding contact with the concave sliding surface (17), the sliding block (16) being capable of sliding with pendulum motion relative to the concave surface (17) in response to an earthquake tremor or shock, the device (11) comprising a sliding material (19) between the concave sliding surface (17) and the convex sliding surface (16 b), where the friction coefficient between the sliding surfaces (16 b, 17) is above 10% when the device (11) is loaded with a pressure above 30 MPa and the sliding surfaces are caused to slide at a sliding speed of more than 50 mm/s at a temperature of 20° C. or more.

Description

The present invention relates to an antiseismic support for supporting civil or construction engineering structures, namely including bridges.
Either term “civil engineering structure” or “construction engineering structure” is used herein to designate civil engineering structures such as buildings or bridges, but also silos and tanks.
Antiseismic support devices are known in the art, which are designed to be installed between the foundation of a civil engineering structure (such as a building or a bridge) and the structure itself, to protect it from an earthquake tremor or shock.
“Pendulum” devices, like the one known from U.S. Pat. No. 4,644,714, are among the devices that accomplish this task: this anti-seismic support comprises a lower support and an upper support, which are joined to the structure to be supported and the foundations of the structure respectively.
Both supports have a respective concave sliding surface and are separated from each other by a slider. The slider has two convex surfaces mating and in contact with the concave surfaces of the upper and lower supports, to form a ball joint.
In the event of an earthquake tremor or shock, the slider can slide with a pendulum motion relative to the first support, thereby protecting the overlying structure from the effects of the earthquake.
The sliding surface of the slider in contact with the concave sliding surface of the first support is usually coated with a sliding material to facilitate mutual sliding of the surfaces.
As is known in the art, a composite material comprising PTFE with carbon fibers or glass fibers is used as a sliding material. The sliding material has a high friction coefficient, up to 20%, which allow dissipation of a large amount of energy during rocking, following an earthquake tremor or shock.
Nevertheless, these devices do not have an adequate wear resistance and for this reason are not suitable for use in supporting structures (such as bridges) that are subject to continuous and frequent in-service movements even when no earthquake occurs, e.g. due to thermal expansion, wind action, or to a sudden change of the load supported by the structure.
In another prior art, according to EP 1 836 404, pendulum anti-seismic bearing devices have been provided, which use a different type of sliding material.
Particularly, according to EP 1 836 404, the sliding material may be unfilled hard PTFE or UHMWPE. These materials exhibit a high wear resistance, and a relatively low friction coefficient, which allows them to accommodate the in-service movements of structure, but prevents them from dissipating large amounts of energy, as might be required in the case of a high intensity earthquake.
In order to use the devices as disclosed in EP 1 836 404 in highly seismic areas, it was proposed to apply viscous dampers to the devices, for dissipating the high energy developed in the event of a strong earthquake shock. This increases the dimensions and costs of the device, due to the parts required in addition to those conventionally used in this kind of supports.
A further drawback of prior art sliding materials is their fast mechanical degradation due to the heat dissipated during the earthquake: at high slip velocities, such as those generated by a high-intensity earthquakes, high friction coefficient and extended seismic excitation, the relative sliding motion of the slider surfaces generates huge amounts of heat, causing fast degradation of the properties of the sliding material.
Therefore, it would be desirable to provide an anti-seismic support that is less exposed to degradation caused by the heat generated by the sliding movement of the slider, while still allowing heat generation by friction, i.e. dissipation of the energy transmitted by the earthquake to the structure.
In the light of the above prior art, the object of the present invention is to provide an anti-seismic support that can solve the above mentioned prior art drawbacks.
Particularly, one object of the invention is to provide an anti-seismic support that can be used to support bridges, e.g. road or railroad bridges, and can withstand high-intensity and long-lasting earthquakes.
A further object of the invention is to provide an anti-seismic support that is not too large, has a simple structure with a small number of parts and is reliable with time.
According to the present invention, these objects are fulfilled by a device as defined in claim 1.
The features and advantages of the present invention will become more apparent from the following detailed description of one practical embodiment, which is given as a non-limiting example with reference to the annexed drawings, in which:

FIG. 1 shows a sectional view of an anti-seismic support of the invention;

FIG. 2 shows an enlargement of the sectional view of FIG. 1.

FIG. 1 shows an anti-seismic support 11 which is designed to be installed among the foundations 13 of a construction engineering structure, and the construction engineering structure 12 itself.
The anti-seismic support 11 is used to support the construction engineering structure and protect it in the event of an earthquake.
The construction engineering structure 12 may be a bridge, e.g. a road or railroad bridge. Nevertheless, the bearing device 11 may be also employed to support buildings or other constructions, such as silos and tanks.
The anti-seismic support 11 comprises an upper support 14 (or first support 14) having a concave sliding surface 17, and a sliding block 16 in sliding contact with the sliding surface 17.
The sliding block 16 has a convex sliding surface 16 b, which contacts the concave sliding surface 17 of the upper support 14.
The sliding block 16 further comprises a convex joint surface 20, facing away from the convex sliding surface 16 b, which is adapted to contact a corresponding concave joint surface 21, formed in a second support 15.
In the figures, the second support 15 is located below the sliding block 16 and will be mentioned hereinbelow as “lower support 15”.
Thus, the anti-seismic support 11 actually comprises two supports 15, 14, which are separated by the sliding block 16. An articulation joint is formed between the sliding block and one of the two supports 15, 14 (15 in FIG. 1), and a slip joint is formed between the sliding block 16 and the other of the two supports (14 in FIG. 1), to absorb earthquake-induced movements.
Obviously, while reference is expressly made in the drawings and the description to a single configuration (with the articulation joint below the sliding surface), the kinematically opposite configuration (with the articulation joint above the sliding surface) is possible and technically equivalent, and shall be deemed to be implicitly disclosed herein.
Likewise, the configuration as shown in FIGS. 2 a and 2 b of patent EP 1 806 404, in which the pendulum anti-seismic support comprises two slip interfaces, instead of a slip interface and an articulation joint, is also deemed to be incorporated herein by reference.
Therefore, if the anti-seismic support comprises two slip interfaces, the considerations relating to the slip interface of the present invention shall be deemed to be equally applicable to both interfaces of the anti-seismic support.
Preferably, the convex sliding surface 16 b and the concave sliding surface 17 have mating curvatures, to allow a pendulum motion of this kind of devices 11; for instance, they may have a spherical shape and hence have the same radius of curvature; alternatively, other configurations may be envisaged, in which at least one of the two surfaces has a variable radius of curvature, for improved centering of the sliding block 16.
The upper support 14 is preferably in the form of a plate and is preferably integrally joined, by known fastening means, to a construction engineering structure to be supported. The upper support 14 is preferably made of steel, but may be also made of aluminum or another material.
During an earthquake tremor or shock, the sliding block 16 is adapted to slide along the concave sliding surface 17 with a pendulum motion.
In the figures, the sliding block 16 is shown in its equilibrium position, i.e. centered relative to the concave sliding surface 17. The sliding block 16 is in this equilibrium position before the earthquake and, depending on the interaction of certain factors, including the type of earthquake, it can recover it at the end of the earthquake.
The bearing device 11 comprises a sliding material 19 that forms the concave sliding surface 17 of the upper support 14 or the convex sliding surface 16 b of the sliding block 16.
Advantageously, the sliding material 19 may be applied to the sliding block 16 or the upper support 14 and may be placed in a matingly shaped seat formed in the sliding block 16 or the upper support 14. Advantageously, the sliding material has a thickness (e.g. a constant thickness) greater than the depth (e.g. a constant depth) of the seat in which it is embedded, thereby projecting out of it.
Alternatively, the sliding material 19 may be applied to the sliding block 16 or the upper support 14 using adhesives or mechanical fastener means, such as screws and/or rivets; in this case, the seat may be omitted.
Preferably, the sliding material 19 is applied to the sliding block 16 and forms the convex sliding surface 16 b of the sliding block 16. The sliding material 19 may be in the form of a sheet of material, e.g. having the thickness of a few millimeters.
The sliding material 19 may include a polymeric material with a polymeric, synthetic, ceramic or metal filler, carbon fiber or glass fiber fillers being excluded from such alternatives.
The sliding material 19 has a relatively high friction coefficient at velocities higher than those typically associated with in-service movements, e.g. movements that displace the elements fixed to the anti-seismic support 11 due to thermal expansion, wind action or variable loads acting upon the surface, or to viscous shrinkage of concrete. The concave sliding surface 17 of the upper support 14 and the convex sliding surface 16 b of the sliding block 16 may slide one upon the other with 9% friction or more, or preferably 10% friction or more, if the device 11 has such a load thereon that an average pressure (defined as the ratio of the vertical load on the device to the projection area on a flat surface of the curved contact surface of the convex sliding surface 16 b of the sliding block 16 and the concave sliding surface 17 of the upper support 14), of 30 MPa exists between the surfaces, with a mutual sliding velocity of 50 mm/s or more.
Therefore, the bearing device 11 is adapted to dissipate a large amount of energy, due to its high friction in case of relatively high slip velocities, similar to those occurring in the event of a high-intensity earthquake.
Furthermore, the sliding material has a high thermal conductivity, of 0.50 W/m*K or more.
This relatively high thermal conductivity value allows fast removal of heat from the contact surface of the sliding block 16 and the upper support 14. In fact, the earthquake energy is dissipated and converted into thermal energy at the interface between the convex sliding surface 16 b of the sliding block 16 and the concave sliding surface 17 of the upper support 14. In the polymer materials that are typically used as a sliding material 19 in the prior art, thermal conductivity is much lower than 0.50 W/m*K. This prevented efficient removal of heat from the contact surface and created peak temperatures in the sliding material 19, possibly degrading the mechanical properties of the material. These degradation effects may include softening of the sliding material 19, which affects its load withstanding capacity and/or reduction of the friction coefficient of the polymer material, leading to a reduced earthquake energy dissipation capacity and reduced performances of the anti-seismic support.
The sliding material 19 as used in this invention is adapted to support high heat generation on the sliding surfaces as compared with prior art materials.
In the case of the present invention, the seismic isolation device is characterized in that it uses, as a sliding material 19, a material having a high thermal conductivity, which allows temperature increase to be limited at the convex sliding surface 16 b of the sliding block 16 and the concave sliding surface 17 of the upper support 14.
Preferably, the thermal conductivity of the sliding material is higher than 0.60 W/m*K, more preferably higher than 0.70 W/m*K, and or lower than 0.90 W/m*K.
The sliding material 19 that is used in the device 11 also has a high wear resistance.
Particularly, the properties of the sliding materials 19 typically undergo the wear test as set forth in EN 15129:2009 for buildings, Paragraph 8.3.1.2.5 and or the US standard “AASHTO LRFD Bridge Design Specification”, Paragraph 15.10.1.2.
Therefore, the anti-seismic device 11 is adapted to also withstand low-velocity movements, such as those that may occur when the device 11 is installed in a structure that is required to accommodate displacements (in-service movements) of the elements fixed to the anti-seismic device 11 during its life, such as bridges.
For instance, in-service movements in bridges may be caused by vibrations due to heavy loads or by thermal expansion and may lead to particularly long slip distances during the life of the bridge.
Preferably, a wear test on the sliding material 19 in which the sliding material 19 has a height of at least 2.0 mm from the plane from which it projects (if it is inserted in an embedded seat) or from the plane on which it is applied (if it is not inserted in an embedded seat), with the application of pressure ranging from 75% to 110% of the design pressure, and at a temperature of 20° C. +8° C. and a velocity of 1 mm/s or more, with a sliding distance of at least 1000 meters, results in the sliding material 19 having its thickness reduced by less than 1.0 mm.
Preferably, subject to the above specification of less than 1.0 mm thickness reduction, the sliding material 19 meets the life test of EN15129:2009 for use in buildings, even in a variant involving a sliding distance of more than 1000 m, more preferably a sliding distance of 1600 m or more.
Preferably, the sliding material 19 comprises PTFE, more preferably the polymer material of the sliding material 19 is substantially or exclusively PTFE.
The use of PTFE as a polymer material affords a relatively low friction coefficient at low slip velocities.
For instance, in friction tests with slip velocities of less than 10 mm/s and with the device 11 loaded with the design load, the friction developed due to the relative sliding motion of the concave sliding surface 16 b of the sliding block 16 and the concave sliding surface 17 of the upper support 14 is less than 7%.
This allows accommodation of low-velocity in-service movements, as possibly required by the structure, such as those caused by wind, thermal expansion, sudden load changes on the structure and viscous shrinkage of concrete.
Preferably, a filler, for instance a metal filler, for instance in powder form is added to PTFE; for example, the filler may be bronze powder.
Preferably, the metal filler is from 30% to 60%, more preferably from 40% to 50% by weight. In a preferred embodiment, the metal filler is about 45% by weight.
The presence of such amount of metal increases the thermal conductivity of the sliding material 19 as compared with the normal conductivity values that can be found in prior art polymer materials.
In the preferred embodiment, in which PTFE is filled with 45% bronze powder, thermal conductivity is higher than 0.7 W/m*K, and is preferably about 0.7 to 0.9 W/m*K.
Preferably, the sliding material is sintered and/or formed into a sheet by compression molding. After molding, the sheet material is processed to the desired thickness.
Preferably, the sliding material 19 as used in the device 11 has a thickness of about 8 mm and projects out of the sliding block 16 by about 3 mm, with about 5 mm thereof being embedded in the sliding block 16.
The sliding block 16 of the figures has a sliding block body 16 a, with a seat formed therein for receiving the sliding material 19. The seat is formed on a surface of the sliding block body 16 a which faces towards the concave sliding surface 17 of the upper support 14. The sliding block body 16 a is advantageously made of a metal material, such as steel or an aluminum alloy.
As an alternative configuration, the sliding material 19 may be applied to the surface of the sliding block 16 in contact with the convex sliding surface 17 of the upper sliding block 14 by adhesives or mechanical fastener means such as screws and/or rivets.
The sliding block body 16 a has a second convex surface 20, facing away from the one on which the sliding material 19 is applied. The surface 20 of the sliding block body 16 a preferably has a spherical shape. The sliding block 16 is rotatably coupled to the lower support 15, thereby forming a ball joint. Particularly, its convex surface 20 is received in a spherical concavity formed in the lower support 15.
Preferably, the surface 20 lies on a layer of sliding material 21, which is mounted on the lower support 15. Preferably, the sliding material 21 consists of a highly-running sheet material, i.e. having a low friction coefficient. For instance, the sliding material 21 may be made from an appropriate known polymer material having a friction coefficient of less than 7%. This joint material may be suitably lubricated with lubricants known in the art, to further reduce its friction coefficient.
Advantageously, in an oscillation test conducted with the sliding block 16 and the upper support 14 moving at a relative velocity of about 200 mm/s, or about 400 mm/s, with at least 70% of the maximum oscillation amplitude admitted by the device 11 and with a nominal load applied thereto, after three oscillation cycles the friction coefficient between the concave sliding surface 17 and the convex sliding surface 16 b (i.e. those with the sliding material 19 interposed therebetween) of the device 11 is 9% or more, preferably 10% or more.
Advantageously, the sliding material 19 maintains a bearing capacity of at least 30 MPa when it reaches a temperature of 200° C.
The bearing capacity may be assessed, for instance, by a compression test conducted on a 155 m diameter disk of sliding material 19, with the method described in “Structural Bearings”, by H. Eggert and W. Kauschle, 2002, in which such 8 mm thick disk is partially embedded in a metal support through 5 mm of its thickness and subjected to constant pressure for 48 hours.
The bearing capacity is the pressure value at which the deformation of the sliding material 19 under load stops within 48 hours and the height of the disk of sliding material 19 is reduced by less than 2.0 mm.
Alternatively, the assessment may be based on the amount of warping of the sliding material 19, which should be, for instance, smaller than 6%, preferably smaller than 4%, e.g. smaller than 2% of its maximum plan size, at the end of the third cycle of the oscillation test as defined above, when conducted at a velocity of 200 mm/s or at a velocity of 400 mm/s.
Furthermore, the sliding material 19 preferably has a softening temperature higher than 250° C.
The above clearly shows that the objects of the present invention have been fulfilled.
The anti-seismic support can provide earthquake protection to structures like bridges, which must accommodate considerable in-service sliding movements even when no earthquake occurs. In fact, the anti-seismic support ensures high resistance to the wear caused by mutual slipping of sliding surfaces.
The anti-seismic support is designed to withstand high-intensity and long-lasting earthquakes, because the high friction coefficients of the sliding surfaces allows dissipation of a large amount of energy.
The anti-seismic device is reliable over time, even during a high-intensity and long-lasting earthquake, as it can adequately dissipate the thermal energy generated by friction, due to the high thermal conductivity of the sliding material 11, which prevents the temperature increase at the interface between the convex sliding surface 16 b of the sliding block 16 and the concave sliding block 17 of the upper support 14 from causing the sliding surface 11 to soften and thus lead to a considerable reduction of its friction coefficient.
Furthermore, the anti-seismic device has a relatively low friction at a low slip velocity, which allows optimized accommodation of in-service movements possibly required by the construction engineering structure.
The sliding material also has a good weather resistance, with very little or no moisture absorption.
Those skilled in the art will obviously appreciate that a number of changes and variants may be made to the arrangements as described hereinbefore to meet incidental and specific needs, without departure from the scope of the invention, as defined in the following claims.
The preferred embodiment of the invention as disclosed herein is a device having spherical sliding surfaces, and symmetric with respect to a vertical axis, in operation. Nevertheless it shall be understood that, in a variant, the bearing device might also have cylindrical sliding surfaces, with a substantially horizontal axis of symmetry, in operation.

Claims

1. An antiseismic device (11) for supporting a civil or construction engineering structure, said device (11) comprising a first support (14) having a concave sliding surface (17) and a sliding block (16) having a convex sliding surface (16 d) in sliding contact with said concave surface (17), wherein said sliding block (16) is capable of sliding with pendulum motion relative to said concave sliding surface (17) in response to an earthquake tremor or shock, the device (11) comprising a sliding material (19) that forms one of:

said concave sliding surface (17) and

said convex sliding surface (16 b),

wherein the coefficient of friction between said sliding surfaces (16 b, 17) is above 9% when the average pressure between said sliding surfaces (16 b, 17) is 30 MPa or more and the sliding surfaces (16 b, 17) are caused to slide at a sliding speed of 50 mm/s or more at a temperature of 20° C. or more,

said sliding material (19) comprising a polymeric material filled with a polymeric, synthetic, ceramic or metal filler, wherein if the the sliding material (19) undergoes the test defined by the Standard EN15129:2009, Section 8.3.1.2.5 for antiseismic bearing devices for buildings or the test defined by the US Standard “AASHTO LRFD Bridge Design Specification”, Section 15.10.1.2, it exhibits less than 1.0 mm thickness reduction at the end of the test.

2. A device (11) as claimed in claim 1, wherein said polymeric material is PTFE.

3. A device (11) as claimed in claim 1, characterized in that the filler is a metal only filler and comprises bronze.

4. A device (11) as claimed in claim 3, wherein the metal filler comprises bronze only.

5. A device (11) as claimed in claim 1, wherein the metal filler is from 30% to 60% by weight of said sliding material (19).

6. A device (11) as claimed in claim 1, wherein the sliding material (19) has a thermal conductivity of more than 0.50 W/m*K.

7. A device (11) as claimed in claim 1, wherein friction between said sliding surfaces (16 b, 17) is less than 7%, when the mutual sliding speed of the sliding surfaces (16, 17) is less than 10 mm/s and the contact pressure between the sliding surfaces (16 b, 17) applied to the device (11) is at least 20 MPa.

8. A device (11) as claimed in claim 1, wherein in an oscillation test of said device (11) in which the sliding speed between said sliding block (16) and said first support (14) is at least 200 mm/s, the oscillation amplitude is at least 70% of the maximum admissible oscillation permitted by the device (11) and the nominal load is applied, during the third oscillation cycle the coefficient of friction between the concave and convex sliding surfaces (16 b, 17) of the device (11) is 10% or more.

9. A device (11) as claimed in claim 1, wherein in an oscillation test of said device (11) in which the sliding speed between said sliding block (16) and said first support (14) is at least 400 mm/s, the oscillation amplitude is at least 70% of the maximum admissible oscillation permitted by the device (11) and the nominal load is applied, during the third oscillation cycle the coefficient of friction between the concave and convex sliding surfaces (16 b, 17) of the device (11) is 10% or more.

10. A device (11) as claimed in claim 1, wherein the sliding material (19) is mounted to the sliding block (16) either as a sheet partially embedded in a corresponding seat formed on the sliding block (16), or as a sheet applied using an adhesive or mechanical fastening means and forms the convex sliding surface (16 b) of the sliding block (16).

11. A device (11) as claimed in claim 1, wherein the sliding material (19) has a softening temperature above 250° C.